Finn Toner provides the latest in our ongoing series of guest features for students. Finn is currently reading an MSc in Mental Health Studies at the Institute of Psychiatry; he blogs at Musings.

Synaesthesia is a condition in which the stimulation of one sense consistently gives rise to an automatic experience in a different sensory modality. These ‘sensory blendings’ are experienced by only a minority of the population, but there have been many famous synaesthetes, especially within the art and music world; for example, Thom Yorke from the band Radiohead apparently ‘sees’ certain musical sounds as colours. The condition is not only interesting in its own right, but several recent findings demonstrate that the study of synaesthesia has the potential to inform our ideas about normal cognition.

It has recently been demonstrated that synaesthetes have unusual neuronal wiring. Using an imaging technique called diffusion tensor imaging, Romke Rouw and Steven Scholte demonstrated that grapheme-colour synaesthetes (graphemes are letters or numbers) have more neuronal connections between a variety of brain areas traditionally associated with visual perception, such as the temporal cortex and fusiform gyrus. This suggests that synaesthesia might result from this abnormal cross-wiring.

However, it has also recently been demonstrated that a transient grapheme-colour synaesthetic experience can be induced in non-synaesthetes, who presumably lack such additional neural connections [pdf]. Using a hypnotic suggestion technique, which is thought to influence the level of neural inhibition, Roi Cohen Kadosh and colleagues reported that the perceptual experience of control participants matched those of congenital synaesthetes. This suggests that synaesthesia might result from the disinhibition of a normal perceptual process; a likely candidate mechanism in this case is disinhibition of feedback, whereby grapheme-induced activation of a polysensory neuron could result in sensory 'leakage' back along the colour perception pathway, and result in the sensation of coloured graphemes. Indeed, such a process could foreseeably happen in a brain area such as the superior temporal sulcus, which is considered an important multi-sensory nexus.

These apparently contradictory findings provide support for the respective traditional theories of synaesthesia - wiring vs. disinhibition - the debate between which has yet to be resolved. However, findings like these also act to emphasise both that we should consider the brain as a functionally interactive and parallel network, and that the resulting neural processes can act in both a bottom-up and top-down fashion. This interactionist perspective on cognition is in stark contrast to the initial Input--Process--Output models proposed at the start of the cognitive revolution.

Specific unusual cases of synaesthesia can also provide interesting insight into normal cognition. In 2007, Daniel Smilek and colleagues reported the case of participant TE, for whom graphemes are experienced as having individual personalities. In an attentional task, they demonstrated that TE fixates significantly longer on graphemes with a negative personality; this implies that she may have difficulty in disengaging her attention from negative graphemes. This example of how synaesthesia can influence overt attention demonstrates that the boundary between the cognitive processes of perception and attention is blurry, again contradicting traditionalist views of cognition.

A final issue is the degree to which synaesthesia is 'normal'. Consider the correspondence between smell and taste: most of us experience a blending of these senses to create the experience of flavour. It is currently debatable as to when such perceptual integration is 'normal' or when it is 'synaesthetic' - perhaps we are all synaesthetes to a certain extent!

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